DESCRIPTION (applicants' abstract) The experiments of this proposal continue studies by the applicants of the mechanism through which particular mRNAs come to be localized in dendrites. This project is the outgrowth of their discovery of a selective positioning of protein synthetic machinery beneath synaptic sites on CNS neurons. They now call these elements synapse-associated polyribosome complexes. The central hypotheses that have guided this work are: 1) that SPRCs synthesize certain key protein constituents of the postsynaptic site, including some of the important functional molecules of the synapse; 2) that this local synthesis is critical for the construction of the synaptic site and for modifying existing synapses; 3) that the synthetic activity of synapse associated polyribosomes is regulated in part by synaptic activity. There is evidence that these processes play a key role in synapse function. Indeed there is emerging evidence that a common human genetic disorder (Fragile X mental retardation syndrome) may cause neuronal dysfunction as a result of disrupting gene expression at the synapse. The current experiments are based on studies of the selective targeting of the transcript of a unique immediate early gene (IEG) to dendrites. This gene, termed activity related cytoskeleton associated protein (Arc) is strongly induced by physiological activity like other IEGs, yet is unique because its mRNA is rapidly delivered throughout dendrites. Even more important, activation of particular synapses causes the mRNA to localize selectively in activated dendritic domains. The Specific Aims are: 1) To characterize the process through which synaptic activity causes mRNAs to localize selectively in activated dendritic segments; 2) To define the synaptic mechanisms (i.e. the receptors that play a role) and the post-receptor signal transduction pathways that underlie the selective targeting of Arc mRNA to particular dendritic domains; 3) To define the address markers within the mRNA that determine the selective targeting; 4) To develop strategies to disrupt the selective localization of mRNA in neurons in vivo so as to allow an assessment of the role of mRNA sorting in neuronal function; 5) To define the RNA Transport Packets (RTPs) in which Arc mRNA is conveyed from the cell body into dendrites and determine whether mRNAs that are targeted to active dendritic domains come to be localized at synapse-associated polyribosome complexes.